An Introduction to the Java Logging API

Unless you've been living under a rock, you already know that the official release of JDK 1.4 came out in the first quarter of this year,
and included with it is a new logging API. This API was first
described in JSR 47.
Essentially the same description is also available in the
documentation of logging for JDK 1.4
.

In addition to the implementation provided in JDK 1.4, there is an
open source implementation of the API for JDKs 1.2 and 1.3 called
Lumberjack,
available on SourceForge. The library provides a way for people to
begin using the API before they make the move to JDK 1.4. It also
allows them to use the API in work they do which must
be done in 1.2 or 1.3 for some reason.

What is the Logging API?

The logging API is part of J2SE as of JDK 1.4,
and it ships with the JDK. It is designed to let a Java
program, servlet, applet, EJB, etc. produce messages of interest to
end users, system administrators, field engineers, and software
developers. Especially in production situations, where things can't
be run in a debugger, or if doing so masks the problem that is
occurring (because it is timing related, for example), such logs
are frequently the greatest (and sometimes the only) source
of information about a running program.

To make sure that logging can be left in a production program, the
API is designed to make logging as inexpensive as possible. To let
code produce fine-grained logging when needed but not slow
the application in normal production use, the API provides mechanisms to
dynamically change what log messages are produced, so that the
impact of the logging code is minimized during normal operation. It
also provides a number of Java interfaces and superclasses that
provide "hooks" for developers to extend the API.

The entire logging API is contained in the package
java.util.logging and is made up of the following
interfaces and classes:

ConsoleHandler

FileHander

Filter

Formatter

Handler

Level

Logger

LoggingPermission

LogManager

LogRecord

MemoryHandler

SimpleFormatter

SocketHandler

StreamHandler

XMLFormatter

We'll examine many of these types in more detail below.

An Example

To help motivate the use of the logging API, we will use a simple
program, based on the idea of a "Hello World" program.

We will expand upon this simple program in order to demonstrate and
motivate the logging API. The first thing we will do is actually
generate a logging message. This is straightforward; we need to
create a Logger object, which represents the
HelloWorld class, like this:

This generates a Logger, which we can use to generate
logging messages. Since you typically use one logger per class, we
can make it a static field of the HelloWorld class. We
use the fully qualified name of the HelloWorld class as
the name of the Logger. We'll look at the reasons for
this in more detail later. Now, let's actually use the
Logger to try to generate a logging message:

The logging API has a configuration file and by default, it provides
a handler that will send logging messages, in the form of a
LogRecord to the console (System.err).
So we would expect to see the string "Hello logging!"; appear
before
our message of "Hello world!". But if you run this program
you only
see "Hello world!":

% java logging.example3.HelloWorld
Hello world

What's going on here? The answer lies in the notion of logging
levels.

Logging Levels

Because we want to leave logging code in an application that goes
into production, we need logging to cost us very little at runtime
when it is not being used. To help achieve this, the logging system
defines a class called Level to specify the importance
of a logging record. The logging library has a set of predefined
logging levels:

SEVERE

The highest value; intended for extremely important messages (e.g. fatal program errors).

WARNING

Intended for warning messages.

INFO

Informational runtime messages.

CONFIG

Informational messages about configuration settings/setup.

FINE

Used for greater detail, when debugging/diagnosing problems.

FINER

Even greater detail.

FINEST

The lowest value; greatest detail.

In addition to these levels, there is a level ALL,
which enables logging of all records, and one called
OFF that can be used to turn off logging. It is also
possible to define custom levels, but in this article we will stick
with the predefined levels.

So now we can explain why our logging record didn't appear. We
used the finest() method of our logger, which generates
a logging record at the Level.FINEST level. By default,
only records at the Level.INFO level or higher
will be logged to the console, so our record wasn't printed. We can cause our logging record to
appear by increasing the logging level used in generating the record, like this:

Of course, using a higher logging level for a logging record is not
always what we want. Sometimes we have messages that are
finer-grained than the INFO level, and we only want
them printed when the information they contain is needed. To allow
this, the logging library allows us to control the log level of our
Loggers.